We provide a continuum of subsidy rules based on a performance indicator that induce a monopoly to choose the socially optimal production level. These subsidy rules result in a reduction of the amount of subsidy paid to the monopolist compared to the standard case where a constant subsidy rate is used. The subsidy rate depends on a state variable that reflects the monopolist's history of performance. This variable depreciates over time, therefore requiring a permanent effort of the monopolist to maintain it at an optimal level. In an example with a linear demand and no production cost, the subsidy costs of inducing efficiency are reduced by almost fifty per cent.